I. Field of the Invention
The present invention pertains generally to the field of wireless communications, and more specifically to detecting, isolating, tolerating, and recovering from loopbacks in wide area networks equipped to process packetized data.
II. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. (As used herein, the term “cellular” systems encompasses both cellular and PCS frequencies.) Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). In particular, IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008, etc. (often referred to collectively herein as IS-95), are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. An exemplary cellular telephone system configured substantially in accordance with the use of the IS-95 standard is described in U.S. Pat. No. 5,103,459, which is assigned to the assignee of the present invention and fully incorporated herein by reference. The aforesaid patent illustrates transmit, or forward-link, signal processing in a CDMA base station. Exemplary receive, or reverse-link, signal processing in a CDMA base station is described in U.S. application Ser. No. 08/987,172, filed Dec. 9, 1997, entitled MULTICHANNEL DEMODULATOR, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In CDMA systems, over-the-air power control is a vital issue. An exemplary method of power control in a CDMA system is described in U.S. Pat. No. 5,056,109, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
A primary benefit of using a CDMA over-the-air interface is that communications are conducted over the same RF band. For example, each mobile subscriber unit (typically a cellular telephone) in a given cellular telephone system can communicate with the same base station by transmitting a reverse-link signal over the same 1.25 MHz of RF spectrum. Similarly, each base station in such a system can communicate with mobile units by transmitting a forward-link signal over another 1.25 MHz of RF spectrum. It is to be understood that while 1.25 MHz is a preferred CDMA channel bandwidth, the CDMA channel bandwidth need not be restricted to 1.25 MHz, and could be any number, such as, e.g., 5 MHz.
Transmitting signals over the same RF spectrum provides various benefits including, e.g., an increase in the frequency reuse of a cellular telephone system and the ability to conduct soft handoff between two or more base stations. Increased frequency reuse allows a greater number of calls to be conducted over a given amount of spectrum. Soft handoff is a robust method of transitioning a mobile unit from the coverage area of two or more base stations that involves simultaneously interfacing with two base stations. (In contrast, hard handoff involves terminating the interface with a first base station before establishing the interface with a second base station.) An exemplary method of performing soft handoff is described in U.S. Pat. No. 5,267,261, which is assigned to the assignee of the present invention and fully incorporated herein by reference.
In conventional cellular telephone systems, a public switched telephone network (PSTN) (typically a telephone company) and a mobile switching center (MSC) communicate with one or more base station controllers (BSCs) over standardized E1 and/or T1 telephone lines (hereinafter referred to as E1/T1 lines). The BSCs communicate with base station transceiver subsystems (BTSs) (also referred to as either base stations or cell sites), and with each other, over a backhaul comprising E1/T1 lines. The BTSs communicate with mobile units (i.e., cellular telephones) via RF signals sent over the air.
In conventional systems, when information is exchanged over the backhaul between a BSC and any of multiple BTSs, or between two BSCs, at a data rate higher than the rate provided by a single E1/T1 link, bit-level inverse multiplexers (IMUXes) are used. One bit at a time, the IMUX segments, or demultiplexes, a high-speed bit stream into fixed quantities and places them onto different E1 or T1 lines. A receiver in the BSC or BTS multiplexes the incoming bit streams and reassembles them into a single high-speed bit stream. The IMUX preserves the bit stream regardless of differential delays from different backhaul connections by adding a segment identifier, which consumes valuable bandwidth. Moreover, the design of bit-level IMUXes typically varies from one manufacturer to another, making it difficult to take advantage of standardized telephone-company interfaces. This is especially significant if different suppliers are used for the BSC and the backhaul interface unit of the BTS. Moreover, for any given E1/T1 link, the bit-level IMUX represents a single point of failure, at least temporarily, for the logical connection between the two communicating entities. It would be advantageous, therefore, to provide a reliable, low-cost method of inverse multiplexing that can be applied to standardized interfaces.
Conventional cellular telephone systems are typically configured for point-to-point transmission of packetized data, such as, e.g., between a BSC and a BTS. One of the main problems that arise when a wide area network is used for point-to-point packet data transmission is the random generation of loopbacks, which prevent point-to-point data communications. Locating the loopback poses a significant problem because of the large size of wide area networks. The term “loopbacks” refers to a condition in which data sent along an E1/T1 line is returned to the sending entity. In one form of loopback, the data might also be transmitted to the receiving entity. In another form, the data is returned to the sending unit without also being transmitted. Loopbacks arise for various reasons including, e.g., physical defects, system testing, and technician error.
In conventional systems loopbacks are detected only after the loss of a significant amount of data. Upon detection, manual intervention is required to locate and eliminate the loopback. A field technician must make various telephone calls and drive out to the site of the loopback. The technician must then manually repair the loopback condition. It stands to reason that detecting and recovering from loopbacks can result in dropped telephone calls and is both labor-intensive, and time-consuming. Thus, there is a need for a built-in method of detecting, isolating, tolerating, and recovering from loopbacks.